1. Field of the Invention
One or more embodiments of the invention are directed to the field of solar tracking systems. More particularly, but not by way of limitation, embodiments of the invention provide systems and methods for keeping an array of heliostat mirrors properly oriented towards a solar receiver.
2. Description of the Related Art
As world oil supplies dwindle, and demand for energy increases, heliostats and other mirror based solar collection devices are becoming a widely utilized method of energy production. A heliostat is a stationary device that tracks the movement of the sun. The heliostat typically contains a mirror that is oriented throughout the day to redirect sunlight towards a receiver. In large-scale solar energy installations, an array of heliostats are arranged to converge the sun's energy onto the receiver. When many heliostats direct sun energy to the same receiver, this receiver is generally referred to as a “central receiver”.
These heliostat arrays generally contain many thousands of mirrors, and maintaining alignment of these mirrors over time towards the central receiver is a problem that is regularly encountered. In some solar installations, various types of mirror orientation devices, such as various actuators, control the orientation of the heliostat mirrors. These actuators are generally motion control devices such as motors, servomechanisms, clockwork mechanisms, and the like that are configured to control the orientation of the heliostat mirror relative to the sun and the receiver. In some instances the actuators are under the control of one or more computers.
In an ideal scenario, each heliostat within the array is positioned to focus the sun's rays onto a central receiver for purposes of heating the central receiver to extremely high temperatures. In many commercial solar installations, the central receiver generally contains a heat receiving medium, such as water or salt. Once heated, the heated medium travels through a heat exchanger, where the heat is used to create steam. The steam in turn may be used to operate a steam turbine and create electrical energy. Alternatively the concentrated solar heat and light can be used to generate electricity by other processes, or use the concentrated solar heat and light to perform other useful tasks.
Using a larger number of heliostat mirrors in an array, all focused onto the same central receiver, typically improves efficiency because more solar energy can be collected and used by the same receiver. As a result, although heliostat arrays can be created using as little as one heliostat mirror and one receiver, typical heliostat arrays contain many thousands of individual heliostats, often arrayed over several or more acres of land.
One problem that negatively impacts heliostat performance is the problem of directing the reflected sunlight from the various heliostat mirrors onto the same desired region of focus on the central receiver throughout the day, and throughout the year. As the time of day varies and as the time of year varies, the angle of the sun in the sky varies, and thus the heliostat mirrors must be continually be repositioned to keep directing a maximum amount of reflected sunlight onto the central receiver. Given that the heliostat mirrors may be located some distance away from the central receiver, even minor errors in mirror orientation can cause the reflected sunlight to miss the receiver, thereby causing the heliostat array as a whole to function with suboptimal efficiency. Although heliostat mirrors are usually controlled by calibrated actuators, the calibration of the actuators may drift with time, causing pointing inaccuracies.
In theory, the orientation of the main beam of reflected sunlight from a given heliostat mirror onto a central receiver can be detected by simply placing sensors on the central receiver, and monitoring the deviation of any given heliostat's main beam from the ideal location on the central receiver. In practice, however, this simple approach is impractical given that as the number of heliostat mirrors increases, the large amount of light and heat on the central receiver will tend to overwhelm (destroy or blind) any sensors placed near the beam focus.
To cope with this problem, heliostat mirror actuators and control systems can also be partially calibrated by a process of orienting one or more heliostat mirrors to an “off sun” orientation (i.e. the mirrors are not directed at the central receiver), calibrating the heliostat actuators in at least the off sun mode, and then returning the heliostat to the “on sun” orientation. However using the man beam from even one off sun oriented heliostat can be problematic because the high intensity of even one main beam can still tend to overwhelm optical sensors and digital cameras, resulting in lower accuracy, and additionally, the off sun mode of the heliostat is at best a surrogate for the heliostat's setting in the on-sun mode.
For at least the reasons set forth above improved methods for positioning heliostat mirrors are desirable.